The field of integrated photonic circuits seeks to create micron-scale photonic elements and to integrate these elements into a single chip-based device. A remaining challenge in integrated photonic circuits is on-chip optical isolation, i.e., the filtering of photons propagating in different directions within the photonic circuit. This challenge can be characterized by the more general goal of providing of non-reciprocal optical elements that can be integrated on-chip (i.e., on a micrometer scale).
Optical isolation can be achieved through the use of magneto-optical properties (e.g., Faraday rotation). However, such an implementation requires large magnetic fields and thus is not amenable for integration on the small scale. Non-magnetic approaches can rely on a dynamical modulation of the index of refraction and/or stimulated inter-polarization scattering based on opto-acoustic effects, modulated dielectric constants and/or optical non-linearities that lead to an intensity dependent isolation.
Such optical isolation schemes may be suitable for certain application. For example, for many commercial applications, high bandwidth and robust fabrication techniques are key requirements. However, requirements can be different in other applications. For example, in on-chip quantum computing, quantum simulation schemes, or quantum Hall physics experiments, it may be desirable to employ optical isolation schemes that provide low losses, operation on a single photon level and the ability to implement coherent non-reciprocal phase shifters, as well as on-chip integration. Many of the above noted optical isolation schemes cannot be easily integrated on chip, while others do not break the time reversal symmetry and therefore, are not suitable for non-reciprocal robust waveguides, or the emulation of real magnetic fields for light.